For psychologist Alain Brunet, the case is still astonishing. When Patrick Moreau first came into his office suffering from post-traumatic stress disorder (PTSD), the Canadian soldier, who had served as a United Nations peacekeeper in Bosnia, could hardly bear to recount the details of the day he was taken hostage in 1993. The memory–of kneeling on the ground with his hands on his head, legs shaking, a stark line of trees across the sky–aroused crippling fear that felt as fresh as it had 15 years before. The glimpse of a particular tree line through his windshield was enough to bring the memory rushing back, giving him such violent shakes that he would have to pull off the road.

But six months after participating in Brunet’s clinical trial, Moreau no longer meets the diagnostic criteria for PTSD. He still experiences some flashbacks, but they are less frequent and less intense. He can now talk calmly and openly about what happened. And all he did was take a blood-pressure drug after writing down the details of the traumatic experience.

“It seemed like science fiction,” says Brunet, a clinical psychologist at McGill University and the Douglas Institute in Montreal. “If someone is traumatized, you ask them to recall the memory, give them a pill, and the [emotional] strength of the memory is weakened.” The details of the trauma remain intact, but the emotional component of the memory appears to dissipate. Although larger studies are needed to assess the potential benefits of the treatment, preliminary findings are promising. Brunet has successfully treated PTSD not only in soldiers like Moreau but also in survivors of rapes and car accidents. “They are matter-of-fact,” he says. “When we ask them whether they have been thinking about the trauma, they raise their shoulders and say, ‘Eh, I am not thinking about it so much.’ It’s like it’s no longer an issue.”

Brunet’s potentially transformative treatment is based in part on a surprising experimental observation: the simple act of calling a memory to mind makes it vulnerable to alteration. Indeed, the right drug given at the right time can make parts of it disappear altogether. If different drugs are delivered to specific parts of the brain, lab animals will explore cages they’ve been conditioned to fear, drink fluids once associated with certain sickness, and ignore sights and sounds that previously led them to expect cocaine or other pleasure-inducing drugs. Humans, too, can be tricked into scrambling their memories in specific ways. For example, if people learn a list of words soon after recalling a previously learned list, they tend to forget the old list or incorporate those words into the new one. The memory of the old list remains intact if people aren’t reminded of it just before learning the new one. And it’s always the old list that gets incorporated into the new, not the other way around.

Brunet and others believe that this phenomenon has to do with a process called memory reconsolidation. The idea is that after someone calls up a memory, it has to be stored in the brain anew. During this process, the memory is in a changeable state. The concept of reconsolidation is still controversial among neuroscientists. But if the theory is correct, and if researchers can figure out just what happens to brain cells and the connections between them when a memory is recalled, it could help answer one of the biggest questions in neuroscience: how memories are physically saved and updated in the brain. It could also explain the malleable nature of memory. “It gives us a new perception of a component of memory we didn’t understand before–how the imperfectness of recall may come about,” says Eric Kandel, a neuroscientist at Columbia University and winner of the 2000 Nobel Prize in medicine.